Lean-burning engines, or engines that run on an air/fuel mixture with a stoichiometrically greater amount of air than fuel, can offer improved fuel economy relative to engines configured to run on stoichiometric air/fuel mixtures.
However, lean-burning engines also may pose various disadvantages. For example, burning a lean air/fuel mixture may decrease the reduction of nitrogen oxides (collectively referred to as “NOx”) in a conventional three-way catalytic converter.
Various mechanisms have been developed to reduce NOx emissions in lean-burning engines. One mechanism is a catalyst known as a NOx trap. The NOx trap is a catalytic device typically positioned downstream of a catalytic converter in an emissions system, and is configured to retain NOx when the engine is running a lean air/fuel mixture for eventual reduction and regeneration when the engine runs a more rich air/fuel mixture.
A typical NOx trap includes one or more precious metals, such as platinum, rhodium, and/or palladium, to convert NOx in an emissions stream to NO2, and an alkali or alkaline metal oxide, such as barium or calcium oxides, to which NO2 adsorbs as a nitrate when the engine is running a lean air/fuel mixture. The engine can then be configured to periodically run a richer air/fuel mixture. The nitrates decompose under these conditions, releasing the NOx which then reacts with the carbon monoxide, hydrogen gas and various hydrocarbons in the exhaust over the precious metal to form N2, thus decreasing the NOx emissions and regenerating the trap.
The use of a NOx trap can substantially reduce NOx emissions from a lean-burning engine. However, during high load conditions, an engine is typically operated at a stoichiometric air/fuel mixture. Under such operating conditions, some hydrocarbons, CO and NOx may reach the NOx trap and be converted to CO2, H2O and N2 at the NOx trap precious metal sites. These conversions are exothermic processes that can age the NOx trap, thereby causing a reduction in NOx storage capacity.
The NOx storage and conversion performance of a NOx trap typically improves with increasing platinum loading at temperatures between 200 and 400° C., particularly after thermal aging. Above 400° C., the influence of the noble metal concentration decreases because the rate of NOx oxidation increases. At these higher temperatures, the NOx storage performance is limited more by the amount of NOx storage sites, which is a function of the volume of the trap. This encourages the use of large NOx trap volumes with high platinum levels in order to provide good NOx storage performance at both low and high temperatures. However, this may increase the cost of the emissions treatment system.